text: To a known volume of sample, an excess but known amount of I is added, which the oxidizing agent then oxidizes to I. I dissolves in the iodide-containing solution to give triiodide ions (I), which have a dark brown color.  The triiodide ion solution is then titrated against standard thiosulfate solution to give iodide again using starch indicator:
  :  (E = +0.54 V)
  Together with reduction potential of thiosulfate:
  :  (E = +0.08 V)
  The overall reaction is thus:
  :  (E = +0.46 V)
  For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis. The disappearance of the deep blue color is, due to the decomposition of the iodine-starch clathrate, marks the end point.
  The reducing agent used does not necessarily need to be thiosulfate; stannous chloride, sulfites, sulfides, arsenic(III), and antimony(III) salts are commonly used alternatives at pH above 8.
  At low pH, the following reaction might occur with thiosulfate:
  Some reactions involving certain reductants are reversible at certain pH, thus the pH of the sample solution should be carefully adjusted before performing the analysis. For example, the reaction:
  is reversible at pH below 4.
  The volatility of iodine is also a source of error for the titration, this can be effectively prevented by ensuring an excess iodide is present and cooling the titration mixture. Strong light, nitrite and copper ions catalyse the conversion of iodide to iodine, so these should be removed prior to the addition of iodide to the sample.
  For prolonged titrations, it is advised to add dry ice to the titration mixture to displace air from the Erlenmeyer flask so as to prevent the aerial oxidation of iodide to iodine.  Standard iodine solution is prepared from potassium iodate and potassium iodide, which are both primary standards:
  Iodine in organic solvents, such as diethyl ether and carbon tetrachloride, may be titrated against sodium thiosulfate dissolved in acetone.